Guided self-assembly of block copolymer films on interferometrically nanopatterned substrates

ABSTRACT

Copolymer structures are formed by exposing a substrate with an imaging layer thereon to two or more beams of selected wavelengths to form interference patterns at the imaging layer to change the wettability of the imaging layer in accordance with the interference patterns. A layer of a selected block copolymer is deposited onto the exposed imaging layer and annealed to separate the components of the copolymer in accordance with the pattern of wettability and to replicate the pattern of the imaging layer in the copolymer layer. Stripes or isolated regions of the separated components may be formed with periodic dimensions in the range of 100 nm or less.

This invention was made with United States government support awarded bythe following agency: NSF Grant No: 9708944. The United States hascertain rights in this invention.

FIELD OF THE INVENTION

This invention pertains generally to the field of nanofabricationtechniques and particularly to nanofabrication carried out utilizingdiblock copolymers.

BACKGROUND OF THE INVENTION

Significant challenges are encountered in the fabrication ofnanostructures, particularly structures at a length scale of 10 nm to 50nm. It is possible to fabricate isolated or semi-dense structures atthis scale with an advanced lithographic technique such as electron beamlithography, but the exposure tools are extremely expensive andoptimization of photo-resist processing is non-trivial and may not beamenable to strict control of dimensions or roughness.

New processes and novel materials are required to make nanofabricationeasier, cheaper, and more versatile. Block copolymers are interestingmaterials for use in nanofabrication because they microphase separate toform ordered, chemically distinct domains with dimensions of 10's of nm.The size and shape of these domains can be controlled by manipulatingthe molecular weight and composition of the copolymer. Additionally, theinterfaces between these domains have widths on the order of 1-5 nm andcan be controlled by changing the chemical composition of the blocks ofthe copolymers. An advantage of using block copolymer systems astemplates is that linewidth, tolerances and margins, and line edgeroughness are dictated by thermodynamics (molecular weight, theFlory-Huggins interaction parameters χ between the blocks of thecopolymer). It is unclear whether standard resist processing, whereperformance depends on control of kinetic processes, will be applicableat the scale of 10′s of nm.

The feasibility of using thin films of block copolymers as templates wasdemonstrated previously by Chaikin and Register, et al., Science 276,1401 (1997). Dense arrays of dots and holes with dimensions of 20 nmwere transferred from a thin film of poly(styrene-b-isoprene) to siliconnitride substrates. The perfection of ordering of domains extend overgrain sizes of approximately 1 μm². For many applications, macroscopicorientation of the copolymer domains over areas as large as several cm²and registration of the domains with the substrate will be required.Thin films with macroscopically ordered domains are envisioned as havingpotential in several applications including nanowires, magnetic storagemedia, quantum devices, and photonic crystals. Strategies for inducingmacroscopic orientation of copolymer domains in thin films haveincluded: (1) the use of electric fields to orient cylindrical domainsin asymmetric diblock copolymer films both parallel to the film alongelectric field lines and perpendicular to the film in hexagonal arrays,(2) the use of miscut silicon wafers as substrates to align thicknessinduced morphologies along the corrugations of the substrate, (3) theuse of miscut silicon wafers with obliquely deposited Au stripes topromote alternating wetting of the blocks on the alternating Si and Austripes and perpendicular orientation of lamellar domains, and (4) theuse of sidewall constraints to induce long range ordering of sphericaldomains in asymmetric diblock copolymers.

One approach to inducing macroscopic orientation of the domains of blockcopolymers combines advanced lithographic techniques and theself-assembly of the block copolymer film. Organic imaging layers arepatterned using lithographic tools, e.g., proximity x-ray lithographywith a mask and extreme ultraviolet (EUV) interferometric lithography.Regions of the imaging layer that are exposed to radiation or electronsundergo a chemical transformation that alters the surface chemistry ofthe imaging layer. A thin film of a symmetric diblock copolymer is thendeposited on the patterned imaging layer and annealed above the glasstransition temperature of the blocks of the copolymer. During annealing,the lamellar domains of the copolymer film self-assemble such thatadjacent regions of the chemically patterned surface are wet by thedifferent blocks of the copolymer. The lamellae orient perpendicular tothe plane of the film and amplify the surface pattern. After annealing,selective removal of one of the blocks results in a nanopatternedtemplate that can be used for additive or subtractive processes fornanofabrication. This strategy has the advantages of achievingmacroscopic orientation of the lamellar domains using parallel exposuretools and registration of the patterned film with the substrate. See,Richard D. Peters, et al., “Using Self-Assembled Monolayers Exposed toX-Rays to Control the Wetting Behavior of Thin Films of DiblockCopolymers,” Langmuir, Vol. 16, 2000 (published on web Apr. 7, 2000),pp. 4625-4631; Qiang Wang, et al., “Symmetric Diblock Copolymer ThinFilms Confined Between Homogenous and Patterned Surfaces: Simulationsand Theory,” Journal of Chemical Physics, Vol. 112, No. 22, Jun. 8,2000, pp. 9996-10010; Tae K. Kim, et al., “Chemical Modification ofSelf-Assembled Monolayers by Exposure to Soft X-Ray in Air,” J. Phys.Chem. B., Vol. 104, 2000 (published on web Jul. 18, 2000), pp.7403-7410.

SUMMARY OF THE INVENTION

In the present invention, advanced interferometric lithography iscombined with self-assembled block copolymer systems to providenanofabricated structures. Interferometry is used to pattern substrateswith regions of different chemical functionality in spatial arrangementscommensurate with the characteristic dimensions of the domain structureof the polymer. Upon ordering, the morphology of a block copolymer layeron the surface of the substrate is guided toward the desired long-rangeorientation, amplifying the pattern on the surface. The block copolymerscan be synthesized for guided self-assembly and either are functional asformed or can be functionalized after microstructures are formed.

In forming the copolymer microstructures in accordance with theinvention, a substrate is provided with an imaging layer thereon thatwill respond to exposure to selected wavelengths to change thewettability of the exposed material of the imaging layer to thecomponents of a selected block copolymer. The imaging layer is thenexposed to two or more beams of radiation within the selectedwavelengths to form interference patterns at the imaging layer to changethe wettability of the imaging layer in accordance with the interferencepatterns. Preferably, the interference pattern in the imaging layer hasa period substantially equal to, and preferably within 20% of, the bulklamellar period L₀ of the selected copolymer. For interference patternshaving periods in the range of 10-100 nm, the exposing radiationpreferably is at selected wavelengths in the extreme ultraviolet orshorter. A layer of the selected copolymer is then deposited onto theexposed imaging layer, and the copolymer layer is annealed to separatethe components of the copolymer in accordance with the pattern ofwettability in the underlying imaging layer to replicate the pattern ofthe imaging layer in the copolymer layer. For example, one of the beamsmay be provided directly from the source onto the imaging layer and theother beam may be provided by reflecting a portion of the beam from thesame source with a Lloyd's mirror onto the imaging layer at an angle tothe beam that is directly incident on the imaging layer. Where two suchbeams are utilized, the resulting pattern in the imaging layer definedby the interference pattern is a periodic pattern of alternating stripeswhich differ in wettability with respect to one of the components of thecopolymer. The resulting microstructure in the annealed copolymer layercan comprise corresponding alternating stripes of the two components ofthe copolymer that are separated in accordance with the regions ofgreater or lesser wettability. Other interferometric processes andinstruments may also be utilized.

A third beam may be provided to the imaging layer by reflecting aportion of the beam from the same source with another Lloyd's mirroronto the imaging layer at an angle to the beam that is directly incidenton the imaging layer. The two Lloyd's mirrors are positioned at an angleto each other to provide interference patterns at the imaging layer thatare at an angle to each other. In this manner, an array of separatedregions can be defined in the imaging layer in which the separatedregions have a higher or lower wettability with respect to one of thecomponents of the copolymer. Deposit of the copolymer layer andannealing of the copolymer layer results in regions of one of thecomponents of the copolymer separated primarily by components of theother copolymer in a pattern which corresponds to the underlying patternin the imaging layer. The imaging layer may be formed of materials suchas self-assembled monolayers such as alkylsiloxanes. Various blockcopolymers may be utilized, one example of which is a copolymer ofpolystyrene and poly(methyl methacrylate).

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a phase diagram of equilibrium film morphologies for symmetricdiblock copolymer films confined between patterned and neutral surfaces.

FIG. 2 is a diagram of interfacial energy between self-assembledmonolayers of OTS exposed at different doses and a copolymer ofpolystyrene and poly(methyl methacrylate).

FIG. 3 is an illustrative diagram of calculated extreme ultravioletintensity at a sample surface utilizing multiple interfering beams.

FIG. 4 is a simplified diagram illustrating the steps in forming ofcopolymer structures in accordance with the invention.

FIG. 5 is a simplified view of an exposure pattern formed on a substrateby three interfering beams.

FIG. 6 is a simplified diagram illustrating the stripe pattern ofillumination at the surface of a substrate from two interfering beams.

FIG. 7 are plots showing the one-dimensional order parameter profilesfor different values of ω_(sA) with α_(sA)=−0.2.

FIG. 8 are graphs illustrating the one-dimensional order parameterprofiles for different values of α_(sA) with ω_(sA)=9.

FIG. 9 are contour plots of the two-dimensional order parameter profilesfor the morphology between a lower hexagonally patterned substrate andan upper sH surface.

FIG. 10 is a schematic diagram of an interferometric lithography systemthat may be utilized to expose samples in a pattern in accordance withthe invention.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, thin films of block copolymer areself-assembled to produce structures that may be used, e.g., astemplates for nanofabrication under appropriately controlled conditions.Processing conditions are preferably utilized that induce perpendicularorientation of the microphase separated domains and amplification of thesurface patterns that are lithographically formed on a substrate. Thefollowing discusses the factors that influence the preferred processingconditions.

The free energy of symmetric diblock copolymer films confined betweenhomogeneous and patterned surfaces is assumed to consist of fourcontributions: (1) the elastic free energy associated with chainstretching, (2) the block—block interfacial energy, (3) thesurface-block interfacial energy, and (4) the free energy of bending ifthe lamellae were not flat. The equilibrium film morphology wasdetermined from proposed structures with the minimum total free energy,and is found to be primarily a result of the competition betweenunfavorable surface-block interactions and chain stretching. Monte Carlosimulations were used that generated all possible structures, and theresults allowed the construction of a phase diagram, as shown in FIG. 1,of equilibrium film morphologies as a function of the film thickness,the period of the pattern on the substrate, the bulk lamellae period L₀of the polymer, and the interaction potentials of the individual blocksof the polymer with the bounding surfaces. This figure shows a phasediagram of equilibrium film morphologies for symmetric diblock copolymerfilms confined between patterned and neutral surfaces. The abscissa isthe ratio of pattern period to lamellar period, and the ordinate is theratio of film thickness to lamellar period. Symbols represent MonteCarlo results, and lines represent results from calculations from thephenomenological model. Schematics (a) through (e) are representationsof the film morphology in each region of the phase diagram, where lightregions represent one block and dark regions represent the other block.The surface pattern is designated by alternating sA and sB regions.Schematic (d) represents the desired film morphology with perpendicularorientation of lamellae that amplify the surface pattern. The optimalconditions that resulted in perpendicular lamellae that amplified thesurface were found to be (1) the period of the substrate pattern equaledthe bulk period of the lamellar domains (L₀), (2) the upper surface ofthe film was near neutral wetting conditions (had equal affinity forboth blocks of the copolymer), and (3) the film thickness was equal toor less than Lo. Slight deviations from these optimal conditions, e.g.,the upper surface had a slight affinity for one of the blocks or thefilm thickness was greater than Lo, still resulted in the desiredbehavior.

Imaging layers were developed to meet the following criteria: (1) thesurface chemistry must be tunable by interferometric techniques suchthat exposed and unexposed regions have different affinities (wetting)for the blocks of the copolymer, and (2) the layers must be thermallystable during a high temperature annealing of the polymer films. Selfassembled monolayers (SAMs) of alkylsiloxanes are suitable as imaginglayers because: (1) the surface chemistry of SAMs of alkylsiloxanes onSiO_(x) can be modified by suitable exposure (e.g., to x-rays in thepresence of air), (2) SAMs are ultrathin layers (2.6 nm) (eliminatingopacity issues for extreme ultraviolet (EUV) lithography), and (3) SAMsof alkylsiloxanes are thermally stable under vacuum up to temperaturesof 740 K. Chemical modification of a CH₃-terminated SAM ofoctadecyltrichlorosilane (OTS) occurs and photoelectrons are emittedfrom the substrate upon irradiation with, e.g., x-rays. Thesephotoelectrons produce radicals at the surface of the SAM. The surfaceradicals react with oxygen in the exposure atmosphere to producehydroperoxy radicals or undergo recombination or crosslinking reactions.The hydroperoxy radicals decompose to form hydroxyl, aldehyde, andcarboxylic acid groups on the surface of the SAM. The selectivity of thereaction for production of hydroperoxy radicals versusrecombination/crosslinking reactions depends upon the pressure of oxygenin the exposure atmosphere, and this competition between reactionsresults in asymptotic conversion of the CH₃ groups to polar, oxygencontaining groups, with the asymptotic limit also being a function ofthe pressure of oxygen.

The wetting behavior of block copolymer thin films has been shown todepend upon the interfacial energy between each block of the copolymerand the substrate, with the block having the lower interfacial energypreferentially wetting the substrate. The interfacial energies (γ)between polystyrene (PS) and poly(methyl methacrylate) (PMMA) and SAMsof OTS exposed to x-rays in air was calculated as a function of dose asshown in FIG. 2, and the wetting behavior of thin films ofpoly(styrene-b-methyl methacrylate) (P(S-b-MMA)) was describedqualitatively at each dose. Unexposed SAMs of OTS were dewet by films ofP(S-b-MMA) because the spreading coefficient was less than 0. Thepolymer films wet SAMs of OTS with the PS block present at the substrateinterface for exposure doses between 300 and 1000 mJ/cm² because γPS/OTSwas lower than γPMMA/OTS. At a dose of 1200 mJ/cm², γPS/OTS=γPMMA/OTS=0,and the surfaces were neutral such that the lamellae orientedperpendicular to the substrate. For doses greater than 1400 mJ/cm²,γPMMA/OTS<γPS/OTS, and the PMMA block preferentially wet the substrate.SAMs of alkylsiloxanes were concluded to be suitable imaging layers: (1)SAMs of OTS exposed to low doses of x-rays in air (between 300 and 500mJ/cm²) are wet by the PS block of P(S-b-MMA) films, (2) SAMs of OTSexposed to high doses of x-rays in air (greater than 1400 mJ/cm²) areconverted to polar surfaces that are preferentially wet by the PMMAblock of P(S-b-MMA) films. Therefore, SAMs of OTS meet the criterionthat exposed and unexposed (or lightly exposed) regions are wet by thedifferent blocks of the copolymer.

The requirements for the exposure system under preferred conditions are:(1) capability to pattern at the scale of L₀ (10-100 nm), and (2)ability to pattern over large areas in a parallel process. EUV,proximity x-rays, and electron beam lithography are capable ofpatterning at the scale of L₀, but only EUV and proximity x-raylithography are parallel techniques. Proximity x-ray lithography islimited by the difficulty of fabricating masks with dense features withperiods at the scale of L₀. EUV lithography is advantageous compared tox-ray lithography because the mean free path of the photoelectronsproduced by photons absorbed in the substrate is minimized for EUVradiation (e.g., λ=13.4 nm). The imaging layer undergoes chemicaltransformation as a result of the photoelectrons emitted from thesubstrate during irradiation. This minimum in the photoelectron energyreduces the lateral spreading of the photoelectrons and results inhigher fidelity in pattern transfer to the imaging layer. Informationregarding the chemical modification of SAMs of OTS with EUV radiation isnot known to the same detail as for x-rays, but exposure of SAMs of OTSto high doses of EUV radiation in air results in similar changes insurface properties such that these surfaces are wet by the PMMA block ofP(S-b-MMA) films.

The present invention carries out lithography to pattern at the scale ofL₀ utilizing interferometric techniques. An exemplary lithography systemin accordance with the present invention is an EUV lithographic systemutilizing a Lloyd's mirror interferometer to produce high densityfringes. Part of the incident beam is reflected from the Lloyd's mirrorat a grazing angle and interferes with the direct beam at the plane ofthe substrate. Such a system is capable of producing grating patternswith very small periods, on the order of 10's of nm. Suitable EUVradiation can be obtained from various sources, including synchrotrons.For a general comparison of presently available EUV sources, see V. Y.Banine, et al., “Comparison of Extreme Ultraviolet Sources forLithography Applications,” Microelectronic Engineering, Vol. 53, 2000,pp. 681-684. Although the present invention is exemplified belowutilizing EUV interferometric lithography, it is understood thatsuitable nano-dimensioned interference patterns may be formed utilizingradiation at longer wavelengths. The following papers discuss examplesof techniques that may be utilized to form nano-interference patterns:T. A. Savas, et al., “Achromatic Interferometric Lithography for100-nm-Period Gratings,” J. Vac. Sci. Technol. B., Vol. 13, No. 6,November/December, 1995, pp. 2732-2735; Xiaolen Chen, et al.,“Interferometric Lithography of Sub-Micrometer Sparse Hole Arrays forField-Emission Display Applications,” J. Vac. Sci. Technol. B., Vol. 14,No. 5, September/October, 1996, pp. 3339-3349; T. A. Savas, et al.,“Large-Area Achromatic Interferometric Lithography for 100 nm PeriodGratings and Grids,” J. Vac. Sci. Technol. B, Vol. 14, No. 6,November/December, 1996, pp. 4167-4170; Wieland Schollkopf, et al., “ACluster Size Nanofilter with Variable Openings Between 2 and 50 nm,” J.of Chemical Physics, Vol. 109, No. 21, 1 December 1998, pp. 9252-9257;J. A. Hoffnagle, et al., “Liquid Immersion Deep UltravioletInterferometric Lithography,” J. Vac. Sci. Technol. B, Vol. 17, No. 6,November/December, 1999, pp. 3306-3309; M. Switkes, et al., “Patterningof Sub-50 nm Dense Features with Space-Invariant 157 nm InterferenceLithography,” Applied Physics Letters, Vol. 77, No. 20, Nov. 13, 2000,pp. 3149-3151.

The wetting behavior and morphology of thin films of P(S-b-MMA) onsurfaces nanopatterned using EUV interferometric lithography werestudied as a function of the commensurability between pattern period andlamellar period. For example, a film of P(S-b-MMA) was deposited on asurface patterned with a period of L₀ (L₀=60 nm) using EUVinterferometric lithography. The film thickness was about equal to L₀.TEM images were obtained with staining of the PS block with RuO₄, sothat the PS block appears darker than the PMMA block, and confirm aperpendicular orientation of lamellae, with the lamellae amplifying thesurface pattern. The lamellae were ordered over distances of severalmicrometers. The experimental conditions were very similar to theoptimal conditions determined by simulations: (1) the pattern periodsubstantially equaled L₀, and (2) the film thickness was ≦L₀. Thecondition of neutral upper surface varied slightly in that the freesurface of the film was slightly preferential to the PS block. Inaddition, the upper surface was a free surface compared to a confinedsurface in the simulations.

It is understood that many other block copolymers may be utilized. Byway of example only, these include poly(styrene-b-isoprene),poly(styrene-b-butadiene), and poly(styrene-b-vinylpyridine).

As discussed above, interferometric patterning is well suited forpreparing substrates to guide the self-assembly of lamellar andcylindrical morphologies because the interference patterns and the blockcopolymer morphologies exhibit the same symmetry. In accordance with theinvention, crossing two coherent beams of light or using aninterferometer such as a Lloyd's mirror interferometer results inalternating constructive and destructive interference “stripes” to guidethe self-assembly of lamellar domains. Double exposure of stripes withan included angle of 120°, or intersecting three equally spaced beams oflight, results in patterns with six-fold symmetry, or spots ofconstructive interference in a hexagonal array, which are utilized forguiding the self-assembly of cylindrical domains. For example, twoLloyd's mirrors can be mounted with an included angle between them of120°. The EUV beam illuminates the edge region, thus being reflected byboth mirrors. This defines three sources (two virtual and one real, thedirect beam). The superposition of the tri-fold symmetric fields createsa triangular exposure pattern of sharp “dots” at the substrate lane,mounted perpendicularly to the symmetry axis of the system. The Lloyd'smirror system design is very tolerant of alignment. The lattice constantof the three-fold pattern can be easily changed by adjusting theincidence angle of the incoming beam. It is also important to noticethat where the maxima of the fringes overlap, a bright spot is formedwith an intensity equal to nine times that of the incoming beam. Thetype of pattern obtained is shown in FIG. 3. The interferometerpreferably is integrated in a small volume, in order to increasestiffness and minimize vibrations. Piezoelectric actuators may be usedto align and maintain the alignment of the mirrors. A similararrangement can be implement with two mirrors at 90°, yielding arectangular pattern. The process is carried out as illustrated in FIG.4. A substrate 10 has an imaging layer 11 formed thereon that ispatterned with EUV as described above. The resulting patterned imaginglayer 13 is then covered with a layer of a selected block copolymer 15,and the copolymer layer is then annealed to form an array of selfassembled separated domains 17 of one polymer that are surrounded by theother polymer 18. Selective functionalization of the patterned structuremay then be carried out, e.g., by etching out or otherwise removing thepolymer in the domains 17 and filling the openings with anothermaterial, for example, a metal such as copper or nickel or a conductingpolymer to form cells 20. The individual cells 20 formed in this mannermay be characterized in various ways, such as by the use of a scanningprobe 22 as shown in FIG. 4.

The following discusses the morphology of thin films of asymmetric(cylinder-forming) diblock copolymers on nano-patterned substrates.Monte Carlo simulations were used to avoid the assumptions implicit intheoretical work. If the substrate is patterned with an array, (e.g., ahexagonal array) of regions as shown in FIG. 5 that are preferentiallywet by the minority component of the block copolymer, and the period ofthe surface pattern is commensurate with the period in the imaginglayer, then the cylindrical domains in the block copolymer film willorient perpendicular to the substrate and amplify the surface patternthroughout the thickness of the film with long-range order.Perpendicular orientation and long-range order of hexagonally packedcylinders in the film can also occur on striped surfaces of the type asshown in FIG. 6, with appropriate pattern dimensions and strengths ofmolecular level interactions between the polymer and the substrate overwhich these desirable configurations are achieved.

To analyze such systems, Monte Carlo simulations were performed in anexpanded grand-canonical ensemble in the framework of a cubic lattice.Rectangular simulation boxes of dimensions L_(x), L_(y) and L_(z) wereemployed. Asymmetric diblock copolymers with chain length N=36 and avolume fraction of the shorter (denoted by A) blocks f_(A)=¼ areconfined between two hard and flat surfaces perpendicular to the zdirection. Periodic boundary conditions are applied in the x and ydirections. We set the reduced temperature to T*≡k_(B)T/∈_(A-B)=1.5,where k_(B) is Boltzmann's constant, T is the absolute temperature, and∈_(A-B)>0 is the repulsive interaction energy between non-bondednearest-neighbor A-B pairs. We also set the reduced chemical potentialto μ*≡μ/(k_(B)T)=46, where μ is the chemical potential of the system.(The ODT temperature of our system is estimated to be around T*=3.0 atthis chemical potential.) Under these conditions, the asymmetric diblockcopolymers form hexagonally packed cylinders in the bulk, with anintercylinder distance L₀≈16.8 (in units of lattice spacing). In ourmodel, one lattice unit represents one Kuhn segment of a real polymerchain (which is about 0.5˜0.7 nm for flexible polymers such aspolystyrene); our system is therefore in the range of molecular weightsthat are suitable for lithographic processes.

The upper surface is homogenous (denoted by sH). To facilitate theformation of perpendicular cylinders, we introduce a slight preferenceby the upper surface for the longer (B) blocks, i.e.,∈_(A-sH)/∈_(A-B)=0.2 and ∈_(B-sH)=0. This energetic preference balancesthe slight entropic preference for the shorter (A) blocks exhibited by ahard and flat surface.

The lower substrate is chemically nano-patterned. The pattern period isset to L_(s)=14(≈{square root over (3)}L₀/2), and the dark stripes areset to strongly prefer the B blocks (denoted by sB), i.e.,∈_(A-sB)/∈_(A-B)=2 and ∈_(B-sB)=0. The film thickness is fixed atD=28(≈{square root over (3)}L₀); this distance is commensurate withparallel orientation of cylinders having bulk dimensions if such amorphology occurs. We also set L_(x)=67(≈4L₀). The morphologicaldependence of the thin films on the width ω_(sA) and the preferenceα_(sA)≡(∈_(B-sA)−∈_(A-sA))/∈_(A-B) of the light stripes (denoted by sA)were then stacked. When the sA stripes energetically prefer A blocks(α_(sA)>0), we set ∈_(A-sA)=0; when they prefer B blocks (α_(sA)<0), weset ∈_(B-sA)=0.

Contour plots were made of the two-dimensional order parameter profiles<ρ_(A)(x,y)−ρ_(B)(x,y)> and <ρ_(A)(y,z)−ρ_(B)(y,z)> for the morphologythat arises between a lower stripe-patterned substrate and an upper sHsurface; here ρ_(A)(x,y), for example, is the function of lattice sitesoccupied by the A segments at a given (x,y) and <> represents theaverage over the z direction for each collected configuration, and thenover all collected configurations (at equilibrium). Suchensemble-averaged profiles are more meaningful and conclusive forcharacterization of different morphologies than visual inspection of afew snapshots of the system configuration. In this case, we setω_(sA)=9, which is commensurate with the diameter of cylinders in thebulk, and set α_(sA)=−0.2 (the same preference as the upper sH surface).Perpendicular cylinders are formed throughout the entire film, and arelocated on the light stripes. Due to the staggered packing of cylinderlayers (in the x-z plane), <ρ_(A)(x,z)−ρ_(B)(x,z)> only exhibits smallvariations around −0.35, which is consistent with the average density(fraction of occupied lattice sites) <ρ>≈0.7 of copolymers in oursimulations. (Such a density corresponds to highly concentratedsolutions or melts of copolymers.).

For the stripe patterns, there is translational symmetry along the xdirection; since there is no preferred location of cylinders along the xdirection, the perpendicular cylinders can oscillate along thisdirection during the course of a simulation run. Each “instantaneous”configuration collected after equilibration shows well-ordered,hexagonally packed perpendicular cylinders. It is anticipated that the“grain” size in this case is larger than that obtained by the use of ahomogeneous substrate, i.e., the long-range ordering of perpendicularcylinders can be obtained by the use of a stripe-patterned substrate.

Similar perpendicular cylinders can be obtained within a large range ofω_(sA), as long as α_(sA)=−0.2. FIG. 7 shows the one-dimensional orderparameter profiles <ρ_(A)(z)−ρ_(B)(z)> for different values of ω_(sA)with α_(sA)=−0.2. the shape of <ρ_(A)(z)−ρ_(B)(z)> reflects theundulation of A-B interfaces in the perpendicular cylinders. However,there is only a small “window” of α_(sA) for obtaining suchperpendicular cylinders. FIG. 8 shows <ρ_(A)(z)−ρ_(B)(z)> for differentvalues of α_(sA) with ω_(sA)=9. A relatively strong preference for thelonger (B) blocks (α_(sA)=−1) leads to two layers of parallel cylinders.On the other hand, even a weak preference for the shorter (A) blocks(α_(sA)=0.5) results in one layer of half-cylinders parallel to thesurfaces and registered with the strip-pattern. Contour plots of thetwo-dimensional order parameter profiles <ρ_(A)(x,y)−ρ_(B)(x,y)> and<ρ_(A)(y,z)−ρ_(B)(y,z)> for a stronger preference for the A blocks,α_(sA)=1, show another layer of parallel cylinders staggered withrespect to a layer of half-cylinders. In this case, the intercylinderdistance (in the plane parallel to the surfaces) is 14, smaller than L₀.Such a frustration may, in addition to the upper sH surface, facilitatethe formation of perpendicular cylinders near the upper surface, wherebulk dimensions are achieved.

With EUV-IL, a hexagonally patterned substrate, as shown in FIG. 5, canbe obtained by double exposure at an angle of 120°, so that the light(sA) regions are regular-hexagonally packed and the rest are sB regions.The sA regions are set to strongly prefer the A blocks, i.e., α_(sA)=2.The stripe width is set along the x and y directions to ω_(sx)=7 andω_(sy)=8, respectively; these values are commensurate with both thedimensions and the hexagonal packing of cylinders in the bulk. The filmthickness is again fixed at D=28(≈{square root over (3)}L₀).

FIG. 9 shows contour plots of the two-dimensional order parameterprofiles <ρ_(A)(x,y)−ρ_(B)(x,y)> and <ρ_(A)(x,z)−ρ_(B)(x,z)> for themorphology between a lower hexagonally patterned substrate and an uppersH surface. It is seen that perpendicular cylinders registered with thesubstrate pattern form throughout the entire film. The maximum values of<ρ_(A)(x,y)−ρ_(B)(x,y)> are about 0.7. The long-range ordering ofperpendicular cylinders is successfully induced by the hexagonallypatterned substrate. In this case the hexagonal pattern is transposedwith respect to those obtained in previous cases, i.e., we have astaggered packing of cylinder layers in the y-z plane. Therefore,<ρ_(A)(x,z)−ρ_(B)(x,z)> is similar to the quantity<ρ_(A)(y,z)−ρ_(B)(y,z)> and <ρ_(A)(y,z)−ρ_(B)(y,z)> now exhibits smallvariations around −0.35. Although the sA regions are rhombus in thiscase, the A domains shown in the figure of <ρ_(A)(x,y)−ρ_(B)(x,y)> arestill circular. This implies that the detailed shape of the sA regionsis not crucial; instead, the arrangement of the sA regions is moreimportant. The patterned substrate obtained by intersecting threeequally spaced beams of light, i.e., spots of constructive interferencein a hexagonal array, would work even better if the spacing between thespots is commensurate with the bulk period.

It is thus seen that both perpendicular and parallel orientations of thecylinders can be obtained by tuning the alternating stripe preferencefor the two blocks. The long-range ordering of perpendicular cylinderscan be induced by a stripe-patterned substrate. An even better orderingcan be obtained by employing a hexagonally patterned substratecommensurate with perpendicular cylinders having the same dimensions andpacking as in the bulk.

The following are examples for purposes of illustrating the invention.Chemical gratings were patterned on SA films of OTS using EUV-IL withperiods from 1400 to 60 nm. The behavior of films of P(S-b-MMA) on thesesurfaces was studied as a function of the ratio of L_(s) to L₀. ForL_(s)>L₀, the surface patterns are transferred into the polymer filmresulting in height variations on the surface. For L_(s)=L₀, the surfacepatterns are amplified by perpendicular lamellae. The two regimes ofbehavior are discussed in terms of the lamellar structure andorientation in the polymer films.

Materials. Polished 100 mm diameter silicon <100> wafers were purchasedfrom Tygh Silicon and used as substrates for deposition of films.Octadecyltrichlorosilane (CH₃(CH₂)₁₇SiCl₃, 95%) was purchased fromGelest and was used as received. Toluene (99.8%, anhydrous), andchlorofrom (99+%, anhydrous) were purchased from Aldrich and were usedwithout further purification. Symmetric poly(styrene-block-methylmethacrylate) block copolymers were purchased from Polymer Source Inc.Two molecular weights of P(S-b-MMA) were used in our experiments: (1)number-average molar mass=51 200 g/mol, polydispersity=1.06, styrenevolume fraction=0.48, and L₀ is about 30 nm; (2) number-average molarmass=143 500 g/mol, polydispersity=1.09, styrene volume fraction=0.51,and L₀ is about 66 nm.

Preparation and Characterization of SA Films of OTS. The silicon waferswere cleaned by immersion in a piranha solution (7:3 (v:v) of 98%H₂SO₄/30% H₂O₂) at 90° C. for 30 min. The silicon wafers wereimmediately rinsed with deionized water (resistivity≧18 MΩ/cm) severaltimes and were blown dry with nitrogen. The cleansed substrates wereimmersed in a 0.1% (v/v) solution of OTS in toluene in a glovebox with anitrogen atmosphere. The typical immersion times used here ranged from24 to 30 h. After the substrates were removed from the silane solution,they were rinsed with chloroform for approximately 30 s, and excesschloroform was allowed to evaporate. The films were rinsed with absoluteethanol and were dried under a stream of nitrogen. Advancing contactangles of deionized water on OTS were measured using a Rame'-Hartgoniometer, and the typical value was about 105±3°. It has been shownthat films of P(S-b-MMA) on the surface of SA films of OTS depositedunder identical conditions exhibited symmetric wetting behavior afterannealing at 180° C. for 24 h.

Patterning of SA Films of OTS by EUV Interferometric Lithography. OTSwas exposed to EUV radiation (λ=13.4 nm) radiation using the EUVinterferometer system at the Center for Nanotechnology in Madison, Wis.,discussed further below. The exposures were carried out in a chamberwith a pressure of 100 mTorr of oxygen. The intensity of the incidentradiation to the surface of SA films was about 10 mW/cm². The period ofthe fringe pattern for all exposures was verified by patterning UV6photoresist (Shipley) after the system was configured with a particulargeometry. Metrology of the photoresist was performed using scanningelectron microscopy (Hitachi 6180 CD). The line and space structures ofphotoresist were often not exactly symmetric, as is the latent image(the interference pattern), but the period does not depend on symmetryand could be accurately determined by averaging over a large number ofstructures.

Deposition of Diblock Copolymer on Patterned OTS. Thin films ofP(S-b-MMA) were deposited onto patterned OTS substrates by spin-coatingfrom dilute solutions (2% w/w) of the copolymers in toluene. Films werespun at rates from 2500 to 3000 rpm for 60 s. The initial thicknesses ofthe films were determined using a Rudolf Research/Auto EL IIellipsometer using a He—Ne laser (λ=632.8 nm) at an incident angle of70° relative to the surface normal of the substrates. The use of theellipsometer to measure the thicknesses of the P(S-b-MMA) films requiredan assumption about the index of refraction, n, for themicrophase-separated material. We assume n was the same as forpolystyrene, and the results were consistent with measurements using aprofilometer. The polymer films were annealed at 180° C. in a vacuumoven for 24 h. After annealing, the films were investigated using atomicforce microscopy (AFM) and transmission electron microscopy (TEM).

The surface topography of the polymer films was characterized using AFM.AFM measurements were performed in air in both contact mode and tappingmode with a Nanoscope III MultiMode system (Digital Instruments). Intapping mode, both topography and phase image were obtainedsimultaneously. The typical imaging force in contact mode was on theorder of 10⁻⁹ N. We used both oxide-sharpened silicon nitride tips(radii=5-40 nm, Digital Instruments) and carbon nanotips (radii<5 nm,Piezomax Technologies, Inc.) to image the topography of the polymersurface.

The internal structure of the films was studied using TEM. TEM wasperformed on a JEOL 200CX at 200 kV in the bright field mode at theMaterials Science Center at the University of Wisconsin-Madison. Sampleswere imaged in plane-view. A layer of carbon (ca. 20 nm thick) wasevaporated onto the surface of films and then covered with a 25% aqueoussolution of poly-(acrylic acid) (PAA). After the sample was dried in airovernight, the P(S-b-MMA)-carbon-PAA composite was peeled off thesubstrate and floated on deionized water with the PAA side down. Afterthe PAA layer dissolved, the floating film was collected onto TEM grids.The films were then exposed to the vapor of the RuO₄ solution for 15min. The RuO₄ selectively stains the PS block and provides contrast inelectron density.

A schematic of the EUV interferometer exposure system utilized for theforegoing example is shown in FIG. 10. An Au-coated polished siliconwafer 30 (prepared by vacuum deposition with a root-mean-squareroughness<0.32 nm) was used as a Lloyd's mirror set at an angle θ to theEUV beam 31 from a radiation source 32 (e.g., a synchrotron lightsource) to reflect part of an incident beam at grazing incidence andinterfere with the direct beam at the sample plane. The fringe period isgiven by λ/(2 sin θ) where θ is the angle of incidence with the Lloyd'smirror. The theoretical limit is 6.7 nm for the minimum printable periodfor incident radiation with a wavelength of 13.4 nm. A silicon wafer 34with an imaging layer 35 of OTS thereon is placed at the downstream endof the mirror 30 so as to irradiate the layer 35 with the directlyincident beam 31 from the source and a beam 37 reflected from the mirror30 which are incident on the imaging layer 35 at an angle to each otherto form interference fringe patterns. The number of fringes depends onthe spatial and temporal coherence of the source. The spatial coherencedepends on the size of the light source and distance from the source. Inthe experimental setup, temporal coherence ultimately limits the numberof fringes, and the source can be considered to be spatially coherentfor practical purposes. The finite temporal coherence limits the allowedoptical path difference between the direct and reflected beams forproducing fringes. The number of fringes, m, is proportional to λ/Δλ.The interferometer can be operated with or without a monochromator (notshown) in the path of the beam before the mirror. With themonochromator, the beam intensity is low, but λ/Δλ is approximately1000. In this mode, approximately 1000 fringes were produced. If thefringe period was 40 nm, for example, the width of the patterned areaobserved in patterns of photoresist was 40 μm. Without themonochromator, the number of the fringes produced is 15-20 (λ/Δλ≈15-20),but the intensity of the beam is approximately 60 times greater thanthat of the monochromatic beam. Because of differences in sensitivityand contrast between photoresist and OTS imaging layers, the number offringes that were observed in patterned photoresist was significantlygreater than the number of fringes that were observed on patterned OTS.Exposures of OTS with grating periods of about 60 nm were performedwithout the monochromator so as to reduce the exposure times toapproximately 50-60 s compared to about 1 h with the monochromator. Thelength of the exposed area corresponded to the width of the beam, about5 mm. The 30 nm features on the patterned SA films of OTS with thesmallest grating periods are the smallest reported features on SA filmsof alkylsiloxanes patterned using a parallel patterning technique.

From an AFM image of the surface of a P(S-b-MMA) film after it wasdeposited and annealed on OTS that had been exposed in the EUVinterferometer, the period of the fringe pattern, L_(s), was found to be900 nm. The initial film thickness of the block copolymer was 66±2 nm(2.2L₀). With knowledge of the initial film thickness and L₀, theformation of topography on the surface of the film and the type oftopography were used to determine the wetting of the block copolymer atthe substrate. For the initial thickness of 2.2L₀, island formation wasindicative of symmetric wetting, and hole formation was indicative ofasymmetric wetting. Three different regions were observed on thissample. The left side of the surface was unexposed because it was in theshadow of the mirror. Islands with heights of about 30 nm (L₀) wereobserved in this region, indicating symmetric wetting of the blockcopolymer with the PS block at the polymer-substrate interface and atthe free surface. The right side of the surface was uniformly exposedbecause this region is outside the area where coherent fringes areproduced. Holes with depths of about 30 nm were observed in the exposedregion, indicating asymmetric wetting of the block copolymer. In thiscase, the PMMA block preferentially wet the polymer-substrate interface,and PS was present at the free surface. On the patterned region in themiddle, the topography of the block copolymer film replicated the periodof the fringes of the EUV exposure. AFM measurements showed that thepattern period was 900 nm, and the difference in height between adjacentregions was 15 nm.

These results may be interpreted as follows. Exposure of OTS to X-raysresults in the incorporation of oxygen-containing functional groups onthe surface of the film. Primary and secondary electrons generated bythe interaction of the X-rays with the substrate are primarilyresponsible for the photochemical reactions. The chemical modificationof OTS exposed to EUV radiation is expected to follow the same mechanismas that for X-ray exposures. The wetting behavior of P(S-bMMA) films onOTS-covered substrates can be tuned from symmetric to neutral toasymmetric wetting with increasing dose of x-rays. The transitions inwetting behavior were correlated to relative values of interfacialenergy for the PS and PMMA blocks on these surfaces. The wettingbehavior observed on OTS exposed to EUV radiation follows the sametrends: symmetric wetting was observed on unexposed OTS or OTS exposedto low doses, and asymmetric wetting was observed on OTS exposed to highdoses. The topography observed in the middle regions is identical tothat observed on OTS patterned with X-ray lithography with dimensions of150-1000 nm. Adjacent exposed and unexposed regions exhibited asymmetricand symmetric wetting, respectively, and the block copolymer filmdiffered in height by 15 nm (½ L₀) across adjacent regions. The heightdifference of ½ L₀ corresponded to the difference in quantized thicknessbetween symmetric and asymmetric wetting behavior.

AFM images of the surface of P(S-b-MMA) films after they were annealedon OTS that had been patterned with EUV interferometer with periods of240 nm and 120 nm showed that the period of the fringe pattern wasreplicated with great fidelity by the undulating topography of thepolymer films. The difference in height between exposed and unexposedregions for both cases was less than 15 nm (½ L₀). To investigate thedecrease in height difference with decreasing pattern dimensions, weconfigured the EUV interferometer to produce Fresnel diffractionfringes. The difference in height between adjacent regions, Δh,decreased from 15 to 3-4 nm, as L_(s) decreased from about 1410 nm(about 47 L₀) to 240 nm (4 L₀, L₀=30 nm).

As the period of the interference pattern fringes with which the OTS wasexposed approached the bulk lamellar period L₀ of the block copolymer, adifferent behavior was observed. An AFM phase image was made ofP(S-b-MMA) film after it was annealed on an OTS surface that had beenexposed using the EUV interferometer configured to produce about 60 nmfringe periods. The initial film thickness was about 60 nm (L₀). AFMimages of the topography of the polymer films revealed that surfaceroughness was on the order of 1 nm, but no pattern in the roughness wasobserved. In phase images, however, a pattern was observed on thepatterned region that matched the fringe period of the exposure. On thenonpatterned unexposed and exposed regions of the samples, there was noevidence of alignment of the features in the phase image.

The foregoing experiment results show that if the period of the stripedpattern in wetting behavior on the surface was greater than L₀, thelamellae structure of P(S-b-MMA) films was orientated parallel tosubstrate and exhibited topography that replicated the underlyingpattern of the surface. The difference in height between adjacentregions had a maximum value of ½ L₀ and decreased as L_(s) approachedL₀. If the period of the surface patterns was commensurate with L_(o),and preferably within 20% of L₀, the lamellae structure was orientedperpendicular to the substrate and amplified the pattern of the imaginglayer. Guided self-assembly of lamellae in block copolymer films wasobserved over macroscopic dimensions.

It is understood that the invention is not confined to the embodimentsset forth herein as illustrative, but embraces all such forms thereof ascome within the scope of the following claims.

What is claimed is:
 1. A method of forming copolymer structures,comprising: (a) providing a substrate with an imaging layer thereon thatwill respond to exposure to selected wavelengths to change thewettability of the exposed material of the imaging layer to thecomponents of a selected block copolymer; (b) exposing the imaging layerto two or more beams of radiation within the selected wavelengths toform interference patterns having a period of 100 nanometers or less atthe imaging layer to change the wettability of the imaging layer inaccordance with the interference patterns in the exposed imaging layer;(c) depositing a layer of the selected copolymer onto the exposedimaging layer; and (d) annealing the copolymer layer on the substrate toseparate the components of the copolymer in accordance with the patternof wettability for the components of the copolymer in the underlyingimaging layer to replicate the pattern of the imaging layer in thecopolymer layer.
 2. The method of claim 1 wherein the beams are atwavelengths in the extreme ultraviolet or shorter and one of the beamsis provided directly incident from a source onto the imaging layer andanother beam is provided by reflecting a portion of a beam from saidsource with a Lloyd's mirror onto the imaging layer at an angle to thebeam that is directly incident on the imaging layer.
 3. The method ofclaim 2 wherein a third beam is provided to the imaging layer byreflecting a portion of a beam from said source with another Lloyd'smirror onto the imaging layer at an angle to the beam that is directlyincident on the imaging layer, the two Lloyd's mirrors positioned at anangle to each other to provide interference patterns at the imaginglayer that are at an angle to each other.
 4. The method of claim 3wherein the two Lloyd's mirrors are positioned at a 120° angle toprovide an array of separated regions that are preferentially wet by oneof the components of the selected block copolymer.
 5. The method ofclaim 2 wherein said source of the beams of radiation is a synchroton.6. The method of claim 1 wherein the imaging layer is an alkylsiloxaneself-assembled monolayer.
 7. The method of claim 1 wherein the imaginglayer is an octadecyltrichlorosilane self-assembled monolayer.
 8. Themethod of claim 1 wherein the selected block copolymer is a copolymer ofpolystyrene and poly(methyl methacrylate).
 9. The method of claim 1wherein the thickness of the copolymer layer is equal to or less thanthe bulk lamellar period L₀ of the selected copolymer.
 10. A method offorming copolymer structures, comprising: (a) providing a substrate withan imaging layer thereon that will respond to exposure to selectedwavelengths to change the wettability of the exposed material of theimaging layer to the components of a selected block copolymer; (b)exposing the imaging layer to two or more beams of radiation within theselected wavelengths to form interference patterns having a period thatsubstantially equals the bulk lamellar period L₀ of the selectedcopolymer at the imaging layer that define an array of separated regionsto change the wettability of the imaging layer in accordance with theinterference patterns in the exposed imaging layer; (c) depositing alayer of the selected copolymer onto the exposed imaging layer; and (d)annealing said selected copolymer layer on the substrate to separate thecomponents of the copolymer in accordance with the pattern ofwettability for the components of the copolymer in the underlyingimaging layer to replicate the pattern of the imaging layer in thecopolymer layer.
 11. The method of Claim 10 wherein the imaging layer isexposed to three beams of radiation within the selected wavelengths toform the interference patterns.
 12. The method of claim 10 wherein thebeams are at wavelengths in the extreme ultraviolet or shorter and oneof the beams is provided directly incident from a source onto theimaging layer and two other beams are provided by reflecting portions ofa beam from the same source with Lloyd's mirrors onto the imaging layerat angles to the beam that are directly incident on the imaging layer,the two Lloyd's mirrors positioned at an angle to each other to provideinterference patterns at the imaging layer that are at an angle to eachother.
 13. The method of claim 12 wherein the two Lloyd's mirrors arepositioned at a 120° angle to provide an array of separated regions thatare preferentially wet by one of the components of the selected blockpolymer.
 14. The method of claim 12 wherein the source of the beams ofradiation is a synchrotron.
 15. The method of claim 10 wherein theimaging layer is an alkylsiloxane self-assembled monolayer.
 16. Themethod of claim 10 wherein the imaging layer is anoctadecyltrichlorosilane self-assembled monolayer.
 17. The method ofclaim 10 wherein the selected copolymer is a copolymer of polystyreneand poly(methyl methacrylate).
 18. The method of claim 10 wherein thethickness of the copolymer layer is equal to or less than the bulklamellar period L₀ of the selected block copolymer.
 19. The method ofclaim 10 further including removing the component of the selected blockcopolymer in the annealed copolymer layer that is in the separatedregions of the array.
 20. The method of claim 19 further includingdepositing a material in openings left in the copolymer layer.
 21. Amethod of forming copolymer structures, comprising: (a) providing asubstrate with an imaging layer thereon that will respond to exposure toselected wavelengths to change the wettability of the exposed materialof the imaging layer to the components of a selected block copolymer;(b) exposing the imaging layer to two or more beams of radiation withinthe selected wavelengths to form interference patterns at the imaginglayer to change the wettability of the imaging layer in accordance withthe interference patterns in the exposed imaging layer and such that theinterference pattern has a period that is within 20% of the bulklamellar period L₀ of the selected copolymer; (c) depositing a layer ofthe selected copolymer onto the exposed imaging layer; and (d) annealingthe copolymer layer on the substrate to separate the components of saidselected copolymer in accordance with the pattern of wettability for thecomponents of the copolymer in the underlying imaging layer to replicatethe pattern of the imaging layer in the copolymer layer.
 22. The methodof claim 21 wherein the beams are at wavelengths in the extremeultraviolet or shorter and one of the beams is provided directlyincident from a source onto the imaging layer and another beam isprovided by reflecting a portion of a beam from the said source with aLloyd's mirror onto the imaging layer at an angle to the beam that isdirectly incident on the imaging layer.
 23. The method of claim 22wherein a third beam is provided to the imaging layer by reflecting aportion of a beam from said source with another Lloyd's mirror onto theimaging layer at an angle to the beam that is directly incident on theimaging layer, the two Lloyd's mirrors positioned at an angle to eachother to provide interference patterns at the imaging layer that are atan angle to each other.
 24. The method of claim 23 wherein the twoLloyd's mirrors are positioned at a 120° angle to provide an gray ofseparated regions that are preferentially wet by one of the componentsof the selected block copolymer.
 25. The method of claim 22 wherein thesource of beams of radiation is a synchroton.
 26. The method of claim 21wherein the imaging layer is an alkylsiloxane self-assembled monolayer.27. The method of claim 21 wherein the imaging layer is anoctadecyltrichlorosilane self-assembled monolayer.
 28. The method ofclaim 21 wherein the selected block copolymer is a copolymer ofpolystyrene and poly(methyl methacrylate).
 29. The method of claim 21wherein the interference pattern in the imaging layer has a period of100 nm or less.
 30. The method of claim 21 wherein the thickness of thecopolymer layer is equal to or less than the bulk lamellar period L₀ ofthe selected block copolymer.